Mr Hammond said: "For Nato to maintain its credibility in the future, we have to demonstrate that we maintain the political will to act to defend ourselves and our interests.

"We have to be clear that we have not lost the appetite to intervene when our interests or our obligations require us to do so.

"If we lack, or are perceived to lack, the political will to respond - and to do so quickly - the credibility of that commitment to collective defence will be undermined, and the very fabric of this, most successful of alliances, will unravel.

"So we have to be clear with Russia, in particular, that while we support a political resolution to the situation in eastern Ukraine, there is a red line around Nato member states themselves that cannot be crossed."